INTRODUCTION TO SONAR 



signals resulting from the echo are amplified 

 and converted to audio signals that can be 

 heard through a loudspeaker or earphones. The 

 receiver also feeds the amplified echo signal 

 to the various video indicating devices, such as 

 the cathode ray tube (CRT) onthe control indicator. 



Searchlight Sonar 



Early active sonars utilized the searchlight 

 principle for transmitting sounds. Like the 

 searchlight, the transducer had to be trained 

 to a particular bearing in order to transmit 

 sound on that bearing. The sound beam was 

 narrow in bearing width (about 5°), consequently 

 the echoes were received from only a small 

 sector of the surrounding sea. An arrangement 

 of this type was necessary at that time because 

 sufficient power for omnidirectional transmission 

 could not be generated. The scanning sonars in 

 widespread use today develop tremendous power — 

 enough to be transmitted 360° in azimuth simul- 

 taneously. 



Late modifications to active sonars allow the 

 selection of directionally transmitted sonic 

 pulses, somewhat related in principle to the 

 earlier searchlight sonars. This feature, called 

 rotating directional transmission, is discussed 

 later. 



The main disadvantage of the searchlight 

 type of sonar was the length of time required 

 to scan the area around the ship. Search pro- 

 cedures called for the operator to transmit, 

 listen for echoes, train the transducer to a 

 new bearing, transmit, listen, and so on, first 

 on one side of the ship, then the other. It was 

 possible for a subm.irine to slip by undetected 

 on the port side, for example, while the operator 

 was searching on the starboard side. Moreover, 

 maintaining contact with a target that had a 

 rapidly changing bearing required a high degree 

 of proficiency on the part of the operator. 

 Another disadvantage was that searchlight equip- 

 ment had only an audio presentation, whereas 

 today's scanning sonars provide both a video 

 and an audio presentation. 



Scanning Sonar 



Modern submarines and ASW ships are 

 equipped with scanning sonar, which transmits 

 sound pulses of high energy in all directions 

 simultaneously. One of the features of scanning 

 sonar is a cathode ray tude (CRT) display of all 

 underwater objects detected in the area sur- 

 rounding the ship. Target echoes appear as 



bright spots on the CRT, similar to the display 

 of a radar's PPI, 



Some of the data that you may learn from 

 the CRT presentation are as follows: 



1. The size of the target may be estimated 

 from the size of the echo. Don't rely too heavily 

 on this feature, though, because echo appearance 

 depends on such factors as target aspect, range, 

 and equipment performance. 



2. The distance of the echo from the center 

 of the CRT represents range to the object 

 from your ship when the CRT is used in the 

 ship center display (BCD) mode. 



3. True bearing of the object can be deter- 

 mined directly on the scope. 



4. Target movement can be determined from 

 its scope presentation. Fixed objects such as 

 reefs and sunken ships will move in a direction 

 parallel to your ship's movement and in the 

 opposite direction. Moving objects may have 

 motion in any direction with respect to own 

 ship. 



5. The wake of a submarine often can be 

 seen. By examining the wake, you may be able 

 to establish a submarine's heading even before 

 its movement can be determined by tracking. 



6. Submarines that are too far away for 

 detection by echo ranging may yet emit enough 

 noise to be detected. Under these conditions, 

 a small segment of the scope appears to be 

 filled with a rippling pattern. The general direc- 

 tion of the noise source can be ascertained 

 by taking a bearing to the center of the noise 

 pattern. 



TRANSDUCERS 



Knowledge of the design and function of the 

 transducer is the key to understanding the prin- 

 ciples of sonar, whether of the scanning or the 

 searchlight type. You already know that a trans- 

 ducer converts outgoing signals from electrical 

 to acoustical energy, and converts incoming 

 signals from acoustical to electrical energy. 

 Signals are converted by the magnetostrlctive, 

 piezoelectric, or electrostrictive process. 



Magnetostrlctive Process 



Magnetostriction is a process whereby changes 

 occur in metals when they are subjected to a 

 magnetic field. If a nickel tube is placed in a 

 magnetic field, for example, its length is shortened 

 as a result of the magnetostrlctive effect. The 

 effect is independent of the direction of the 

 magnetic field. 



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